270 EVOLUTION, OLD AND NEW. 



to this illustration, stands at the extremity of each 

 bough, should bear a resemblance, at least upon one 

 side, to the other neighbouring species ; and this cer- 

 tainly is what we observe in nature. 



" Having arranged living forms in such an order as 

 this, let us take one, and then, passing over several 

 boughs, let us take another at some distance from it ; 

 a wide difference will now be seen between the species 

 which the forms selected represent. Our earliest col- 

 lections supplied us with such distantly allied forms 

 only ; now, however, that we have such an infinitely 

 greater number of specimens, we can see that many of 

 them blend one into the other without presenting note- 

 worthy differences at any step." * 



This has been well extended by Mr. Darwin in a 

 passage which begins : — " The affinities of all beings 

 of the same class have sometimes been represented by 

 a great tree. I believe that this simile largely speaks 

 the truth." t 



" What, then," continues Lamarck, " can be the 

 cause of all this ? Surely the following : namely, that 

 when individuals of any species change their situation, 

 climate, mode of existence, or habits [conditions of life], 

 their structure, form, organization, and in fact their 

 whole being becomes little by little modified, till in 

 the course of time it responds to the changes experi- 

 enced by the creature." % 



In his preface Lamarck had already declared that 



" the thread which gives us a clue to the causes of the 



■■ 'Phil. Zool.,' torn. i. pp. 75-77. 



t ' Origin of Species,' p. 104, ed. 1876. 



X <Fhil. Zool.,' torn. i. p. 79. 



